1. Field of the Invention
The present invention relates, in general, to internal combustion engine control systems and, specifically, to an internal combustion engine control system capable of controlling the air-to-fuel ratio upon a cold start of the engine.
2. Description of the Art
When an engine is cranked upon startup, the engine controller compares the value from the coolant temperature sensor with values stored in a lookup table to determine the correct air/fuel (A/F) ratio at that temperature. Typically, the fuel control system provides an A/F ratio of between 2:1 to 12:1, depending on the temperature. Upon a cold start, the controller switches to an open-loop run mode shortly after engine crank mode is complete. The A/F ratio during open-loop run mode is rich and is also determined from a lookup table incorporating start-up coolant temperature. The controller continues this open-loop run mode until the engine is warmed up. Generally, the engine is warmed up when it reaches a total warm-up time derived from a look-up table based on the start-up coolant temperature and current coolant temperature. When the engine is warmed up, or immediately upon a warm start, the controller switches to closed-loop fuel control, assuming the exhaust gas oxygen sensor is sufficiently warmed. If the controller is unable to switch to closed-loop fuel control, open-loop fuel control continues using an A/F ratio of close to stoichiometric, 14.7.
Currently, the lookup table used for open-loop run mode is calibrated to a fuel with a high driveability index (xe2x80x9cDIxe2x80x9d). Driveability index is an indicator of the amount of heat required to evaporate a particular fuel. The higher the DI, the more heat is required to evaporate the fuel. The majority of fuels have a DI from 1100 to 1150, however, the calibration of the lookup table typically is performed using a xe2x80x9cworst case fuel of 1250 DI. As a consequence of calibrating the lookup table to a fuel with such a high DI, the engine is required to run richer than required if the lookup table was calibrated to a fuel with a better driveability index. This rich mixture facilitates a rapid, smooth start-up regardless of the DI of the fuel. If insufficient fuel is used during this period, engine misfires or stalls could result.
Because the catalytic converter does not begin processing emissions from the engine until the converter reaches an appropriate operating temperature and exhaust has excess oxygen to oxidize HC and CO, this additional fuel translates directly into higher emissions, specifically HC (hydrocarbon) and CO (carbon monoxide) during open-loop run mode. One solution to this problem is to heat up the converter faster. For example, an air injection reaction system injects air into the exhaust to produce an exotherm and thereby raises exhaust gas and converter temperature. This increases the operating temperature of the engine rapidly and adds excess oxygen to the exhaust, raising exhaust gas and converter temperature. However, the system requires the addition of an air pump and plumbing, increasing engine expense complexity.
The present invention provides a method and apparatus that allows leaner initial operation of an engine during open-loop run mode. The present invention recognizes that calibrating the lookup table used in determining fuel supplied during the open-loop run mode to a fuel with a low driveability index (xe2x80x9cDIxe2x80x9d) will provide indicators to the engine of inadequate fueling if the engine is using a fuel with a high DI. Therefore, the invention uses an open loop A/F lookup table calibrated to a fuel with a low DI and monitors certain engine indicators of fuel problems, such as the engine misfire detection system, engine speed and/or manifold absolute pressure during open-loop run mode to determine if additional fuel is needed.
Specifically, the method of the present invention controls the air-to-fuel ratio in an internal combustion engine during open-loop run mode by: fueling the engine to an air-to-fuel ratio based on a fuel with a low driveability index, preferably 1100-1150 DI; monitoring at least one engine indicator to detect a problem with the air-to-fuel ratio; and increasing a level of fuel supplied to the engine to a maximum fuel level when the monitoring step shows a problem with the existing air-to-fuel ratio. In one aspect of the invention, the maximum fuel level is determined based on a fuel with a high driveability index, preferably 1250 DI.
The apparatus of the present invention controls the air-to-fuel ratio in an internal combustion engine during open-loop run mode, using means for fueling the engine to an air-to-fuel ratio based on a fuel with a low driveability index; means for monitoring at least one engine indicator to detect a problem with the air-to-fuel ratio; and means for increasing a level of fuel supplied to the engine to a maximum fuel level when the monitoring step shows the problem with the air-to-fuel ratio. predetermined threshold speed, or when the manifold absolute pressure rises above a predetermined threshold pressure. Alternatively, any one of these systems could be used to detect a problem.
In another aspect of the invention, increasing the level of fuel supplied to the engine to a maximum fuel level takes place in more than one incremental step.
By fueling the engine based on a fuel with a low DI during open-loop run mode, the present invention is intended to result in leaner operation of most engines during open-loop run mode, resulting in a reduction of HC emissions produced by the engine. Since relatively few vehicles are supplied with a fuel with a high DI, additional fueling events as a result of this change are expected to be minimal.